High Memory Support on systems >1GB

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High Memory Support on systems >1GB

John A. Sullivan III
On a Xen 2.0.7 host with between one and four GB of RAM but no single
domain larger than 880MB, how should kernel High Memory Support be
configured? Should it be off since no domain kernel is using more than
880? Should dom0 have it enabled to manage all the memory? Should all
the domains have it since they are drawing on a pool larger than 880 MB?

Is the answer the same for Xen 3.0? Thanks - John
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Re: High Memory Support on systems >1GB

Ernst Bachmann
On Friday 13 January 2006 05:55, John A. Sullivan III wrote:
> On a Xen 2.0.7 host with between one and four GB of RAM but no single
> domain larger than 880MB, how should kernel High Memory Support be
> configured? Should it be off since no domain kernel is using more than
> 880? Should dom0 have it enabled to manage all the memory? Should all
> the domains have it since they are drawing on a pool larger than 880 MB?
>
> Is the answer the same for Xen 3.0? Thanks - John

Since the xen hypervisor needs the same highmem setting as the kernels running
on it, you need the following settings to use all your ram:

- 1 GB: no highmem needed
1-3 GB: 3GB higmem
4GB: PAE or 64bit

and yes, its the same for xen3

/Ernst

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Re: High Memory Support on systems >1GB

Mark Williamson
In reply to this post by John A. Sullivan III
> On a Xen 2.0.7 host with between one and four GB of RAM but no single
> domain larger than 880MB, how should kernel High Memory Support be
> configured? Should it be off since no domain kernel is using more than
> 880?

Xen will support all your memory (unless there's a big memory hole, causing
some of it to be mapped above 4Gig).  Domains only need highmem4g if they're
going to individually use more than 880.

> Should dom0 have it enabled to manage all the memory?

Nope, Xen manages all the memory, so you should be fine.

> Should all  
> the domains have it since they are drawing on a pool larger than 880 MB?

Nope.  But...

*if* you need PAE, then Xen and all domains do need to use PAE.

> Is the answer the same for Xen 3.0? Thanks - John

Yes. (and PAE is Xen 3 only).

Cheers,
Mark

--
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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Re: High Memory Support on systems >1GB

Rene Marticke
In reply to this post by Ernst Bachmann
Ernst Bachmann schrieb:

> On Friday 13 January 2006 05:55, John A. Sullivan III wrote:
>  
>> On a Xen 2.0.7 host with between one and four GB of RAM but no single
>> domain larger than 880MB, how should kernel High Memory Support be
>> configured? Should it be off since no domain kernel is using more than
>> 880? Should dom0 have it enabled to manage all the memory? Should all
>> the domains have it since they are drawing on a pool larger than 880 MB?
>>
>> Is the answer the same for Xen 3.0? Thanks - John
>>    
>
> Since the xen hypervisor needs the same highmem setting as the kernels running
> on it, you need the following settings to use all your ram:
>
> - 1 GB: no highmem needed
> 1-3 GB: 3GB higmem
> 4GB: PAE or 64bit
>
> and yes, its the same for xen3
>
> /Ernst
Hi, I compiled xen 2.07 with the pae part in make world.
But i get only 830MB on a DomU (mem = 2048 in configfile)

I also recompiled the Dom0 Kernel. But i get no success. Can someone
discribe this i bit closer please how to get 2 GB Ram in a DomU ?

Best Regards
Rene


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Re: High Memory Support on systems >1GB

Mark Williamson
In reply to this post by Ernst Bachmann
> Since the xen hypervisor needs the same highmem setting as the kernels
> running on it, you need the following settings to use all your ram:

Nb. only necessary to build the hypervisor and guests with the same setting in
the case of PAE.  Non-PAE Xen supports both highmem4g kernels and normal
kernels, alongside each other, because they still use the same pagetable
format.

Cheers,
Mark

> - 1 GB: no highmem needed
> 1-3 GB: 3GB higmem
> 4GB: PAE or 64bit
>
> and yes, its the same for xen3
>
> /Ernst
>
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Re: High Memory Support on systems >1GB

John A. Sullivan III
In reply to this post by Mark Williamson
On Fri, 2006-01-13 at 14:38 +0000, Mark Williamson wrote:

> > On a Xen 2.0.7 host with between one and four GB of RAM but no single
> > domain larger than 880MB, how should kernel High Memory Support be
> > configured? Should it be off since no domain kernel is using more than
> > 880?
>
> Xen will support all your memory (unless there's a big memory hole, causing
> some of it to be mapped above 4Gig).  Domains only need highmem4g if they're
> going to individually use more than 880.
>
> > Should dom0 have it enabled to manage all the memory?
>
> Nope, Xen manages all the memory, so you should be fine.
>
> > Should all  
> > the domains have it since they are drawing on a pool larger than 880 MB?
>
> Nope.  But...
>
> *if* you need PAE, then Xen and all domains do need to use PAE.
>
> > Is the answer the same for Xen 3.0? Thanks - John
>
> Yes. (and PAE is Xen 3 only).
>
> Cheers,
> Mark
>
Thanks very much.  So just to make completely sure I understand, on a
physical host with between 880 MB and 4 GB of RAM, if no single domain,
privileged or unprivileged requires more than 880 MB of RAM, no domain
kernel needs high memory support enabled.  If any domain, privileged or
unprivileged, will use more than 880 MB, then that kernel and only that
kernel needs high memory support enabled.  Correct? - John
--
John A. Sullivan III
Open Source Development Corporation
+1 207-985-7880
[hidden email]

Financially sustainable open source development
http://www.opensourcedevel.com


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Re: High Memory Support on systems >1GB

Mark Williamson
> Thanks very much.  So just to make completely sure I understand, on a
> physical host with between 880 MB and 4 GB of RAM, if no single domain,
> privileged or unprivileged requires more than 880 MB of RAM, no domain
> kernel needs high memory support enabled.  If any domain, privileged or
> unprivileged, will use more than 880 MB, then that kernel and only that
> kernel needs high memory support enabled.  Correct? - John

Yep, that should work fine.  And you don't need to do anything special to Xen,
it doesn't have the same concept as Linux regarding highmem.  Dom0 doesn't
need highmem to manage the memory, because mapping other domains' memory is
special-cased and not managed in the normal kernel way.

The "all or nothing" thing is only for PAE.

CHeers,
Mark

--
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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